The molecular events involved in the initiation and regulation of muscular contraction are studied with in vitro systems, using the nanosecond fluorescence and other physical techniques. The regulatory proteins from both vertebrate and invertebrate muscle are examined in terms of their interactions with one another and with actin and myosin. The role of actin in the regulatory process is studied by using a fluorescent cofactor of this protein to examine the relationship of its conformation to the contractile process. In connection with the use of the nanosecond polarization method to deduce information on macromolecular flexibility, computer simulation studies are carried out for non-rigid structures, relating rotational Brownian motion to fluorescence polarization.